Providing a drain heat exchanger is a known technique for improving the efficiency of an absorption refrigerating machine using vapor as a heat source. The drain heat exchanger is disposed so as to perform heat exchange between vapor drain, which has heated and concentrated a solution in a high-temperature regenerator, and an absorption solution. Japanese laid-open patent publication No. 51-11332 discloses an example of this kind of drain heat exchanger provided between a low-temperature solution heat exchanger and a high-temperature solution heat exchanger. In this example, a dilute solution is discharged from the drain heat exchanger with an increased temperature, and therefore sufficient heat recovery cannot be performed in the high-temperature solution heat exchanger. Japanese laid-open patent publication No. 51-13259 discloses a technique to solve the above drawback. This technique introduces a concentrated solution, which has been heated in a low-temperature solution heat exchanger, into a drain heat exchanger and a high-temperature solution heat exchanger in parallel and then into a high-temperature regenerator.
However, the above conventional technique may meet another problem when increasing the temperature efficiency of the high-temperature solution heat exchanger and the drain heat exchanger in order to perform sufficient heat recovery and to further improve the efficiency of the absorption refrigerating machine. Specifically, the solution may flash in the high-temperature solution heat exchanger and the drain heat exchanger, causing corrosion. Further, flashing of the solution would prevent flow of the solution through the high-temperature solution heat exchanger and the drain heat exchanger, and therefore result in a lowered heat-transfer capability.
This problem may remarkably occur in a case where a path of the dilute solution to be supplied to the high-temperature regenerator is divided into two branches. This is because amounts of the dilute solutions flowing respectively through the high-temperature solution heat exchanger and the drain heat exchanger become small, and therefore the temperature of a dilute-solution side increases greatly, exceeding a saturation temperature that is determined by the concentration and the pressure of the dilute solution.
In order to solve this problem, it has been proposed to provide a resistance such as an orifice so as to provide a back pressure at outlet sides of the high-temperature solution heat exchanger and the drain heat exchanger. However, this approach may cause another problem because it would require an increased pump power by an amount corresponding to the resistance of the orifice.
Further, in an absorption refrigerating machine having an evaporator and an absorber designed to operate in a multistage manner, the concentration of the solution at an outlet of the absorber is considerably lowered, and hence a temperature at which the solution flashes is lowered according to the decrease in concentration.
Recently, in order to meet customer requirements, there has been a tendency in a production lineup of the absorption refrigerating machine to provide two types, a standard type and a high-efficiency type.
The standard type is provided as a basic model designed to prevent flashing due to an increase in temperature of the solution by restricting the temperature efficiency of the high-temperature solution heat exchanger and the drain heat exchanger. Additional functions for increasing efficiency can be added to the standard type, so that high efficiency can be easily achieved and productivity can be further increased.